1. Field of the Invention
The present invention concerns a magnetic resonance acquisition system of the type having an antenna arrangement with a number of receiver antennas to receive MR response signals. The invention also concerns a receiver antenna module of the type having at least one group of receiver antenna elements to receive the MR response signals, and at least one combination network associated with the group of receiver antenna elements in order to combine the MR response signals received by the receiver antenna elements to form modes. Furthermore, the invention concerns a transmission signal receiver module of the type having a channel switching matrix; with a number of input channels for connection with transmission lines of a transmission line arrangement; a number of output channels; and a control input to activate the channel switching matrix in order to connect a transmission signal received at one input channel to an output channel to relay the transmission signal to an input of an MR signal processing device. The invention also concerns a magnetic resonance system with such a magnetic resonance acquisition system as well as a method to transfer MR response signals from an antenna arrangement with a number of receiver antenna elements to an MR signal processing device, of the type wherein initially MR response signals received by different receiver antenna elements are combined into modes and these modes are then transferred to the MR signal processing device.
2. Description of the Prior Art
In a magnetic resonance examination, local coils are predominantly used to receive the magnetic resonance (MR) response signals. These local coils are receiver antenna modules that contain at least one, but often multiple, receiver antenna elements, generally in the form of conductor loops. The received MR response signals are normally pre-amplified while still in the local coil and are conducted out of the central region of the magnetic resonance system via cables and supplied to a shielded receiver of an MR signal processing device. In the processing device, the acquired data are then digitized and processed further. In many examinations, a number of such local coils are arranged on the patient in order to cover entire regions of the body of the patient. It therefore often occurs that the number of the antenna elements to be used during an examination exceeds the number of input channels that are present at the MR signal processing device, or the number of receiver modules of such an MR signal processing device. It is normally the case, however, that all antenna elements are not simultaneously used during such a measurement, since often only those antenna elements located in a specific region in which a magnetic resonance excitation is presently occurring actually receive MR response signals. Receiver elements whose field of view would not encompass the body region from which MR signals originate during a specific measurement sequence are normally deactivated. In order to thus connect multiple receiver antenna elements to an MR signal processing device that has only a lower number of input channels, a channel switching matrix (also more often called an RCCS: Receive Coil Channel Selector) is therefore used at the MR signal processing device. This switching matrix can be activated so that the currently active receiver antenna elements, or the outputs of the local coils that belong to these antenna elements, are connected to the available input channels of the MR signal processing device. The typical active elements are predominantly those elements that are situated in the homogeneity volume of the magnet.
In order to be able to use the same receiver system with different MR signal processing devices that respectively have different numbers of input channels, or to more easily, retroactively expand the number of receiver channels in a magnetic resonance system, a method to transfer MR response signals of the aforementioned type is described in DE 103 13 004 A1, in which, initially, the MR response signals received by different receiver antenna elements are combined into combinations are known as modes, that are then transferred to the MR signal processing device. A “mode” in this context means a distribution of the sensitivity in the antenna field as a function of space (spatial coordinates). A mode is thus an analog output signal that is formed as a linear combination of the MR response signals of the different receiver antenna elements that are used. This linear combination can be formed in a combination network arrangement composed of a number of combination networks (known as mode matrices). A mode matrix is, for example, a circuit formed of phase shifters and hybrids that combine the signals according to magnitude and phase so that n linear, independent modes can be generated from n input signals. Such a linear combination can be, for example, the sum of multiple MR response signals, wherein each MR response signal, phase-shifted and weighted, can be entered into the sum. A sum of, for example, two signals with a phase difference of 180° corresponds to a difference signal of the two signals. In an extreme case of a linear combination, all signals except for one, with a weight of 0, can enter into the linear combination, such that the resulting mode corresponds to the one MR response signal that was possibly still phase-shifted. In the normal case, however, at least two MR response signals are linearly combined with one another. The modes are preferably formed so that at least one primary mode is formed that already offers the most important image information and the maximum signal-to-noise ratio in the center of the current measurement field of view (FOV). The use of the higher modes then increasingly offers an improvement of the signal-to-noise ratio in the peripheral regions of the FoV and in particular enables the application of parallel imaging techniques. The mode matrices or the combination network arrangement are/is located in the individual local coils, such that instead of the MR response signals, the modes formed thereby can be transferred via cables to the input channels of the MR signal processing device. By the interconnection of the typical channel switching matrix (RCCS), the possibility then exists to decide whether all signal information is read out by a lower number of local coils in order to use these for a parallel imaging technique, or whether instead at least the primary mode is read out by a larger number of receiver antenna elements in order to thus graphically acquire an optimally large spatial region and nevertheless to obtain a relatively good signal-to-noise ratio in the most important regions.
The number of transmission channels from the local coils to the MR signal processing device is not reduced by this method. As cited above, these transmission lines normally run in shielded cables, and the number of such cables that occur with increasing number of receiver antenna elements or local coils is an ever-increasing problem, since the cable bundles that are thereby formed are very unwieldy and are additionally exposed to increased mechanical stresses due to the movement of the patient bed.